1. Field of the Invention
The invention relates to a deflection unit for a cathode-ray tube, comprising a first and a second system of deflection coils and an annular core of a sintered, oxidic, ferromagnetic material. The cathode-ray tube for which the deflection unit is destined may be, for example, a display tube (for example, a data graphic display tube, a colour display tube or a projection television display tube), an oscilloscope tube or a camera tube.
2. Description of the Related Art
A deflection unit for camera tubes usually has a cylindrical core, whereas deflection units for display tubes generally have an annular core which is cup-shaped or funnel-shaped. For mechanical rigidity, annular cores of sintered oxidic ferromagnetic material for display tubes, hereinafter also termed yoke rings, are generally given a wall thickness which is larger than is necessary for the actual function as a core material for deflection coils. For example, a given type of ferrite yoke ring for black-white display tubes has a mass approximately 360 g, height approximately 55 mm, upper and lower diameters approximately 55 and approximately 85 mm, respectively (measured internally), and a wall thickness of 6 mm, a value which with a view to the said properties is unnecessarily high. 6 mm is a frequently occurring wall thickness in ferrite yoke rings. Such deflection units are secured to the necks of display tubes by means of a clamping band, as a result of which the neck of the tube supports the whole weight of the deflection unit. Therefore, too high a weight is disadvantageous since fracture of the neck of the tube is possible. The existing practice of overdimensioning ferrite yoke rings as regards their wall thicknesses is based on the assumption that in this manner mechanical rigidity is achieved which is necessary for handling during manufacture as well as during the mounting in the deflection unit. This assumption in turn is associated with the lack of insight as regards the factors determining the mechanical rigidity of a ferrite yoke ring. An essential problem hence is: how do we obtain a yoke ring of a sintered, oxidic ferromagnetic material having a reduced wall thickness while maintaining its mechanical rigidity. The availability of a yoke ring having a reduced thickness is also of importance for deflection units of camera tubes. In practice, only yoke rings in the form of non-metal sleeves or rolls have so far been used in such deflection units, because a yoke ring of ferrite was said to occupy too much space.